1. Field of the Invention
The present invention relates to a wall structure using a bearing wall panel for houses built by a timber framework construction method.
2. Description of Related Art
Conventionally, it is prescribed that when designing the structure of a building, the design be such that the building as a whole is safe in terms of structural resistance to its own weight, live load, accumulated snow, wind pressure, earth pressure, and water pressure; and earthquakes and other vibrations and impacts by effectively arranging pillars, beams, floors, walls, and the like so as to withstand certain levels of wind force and seismic force.
Moreover, it is prescribed that in a building in which walls, pillars, and horizontal members are made of wood, frameworks having a wall or a brace be arranged in a well-balanced manner in a span direction and a ridge direction on each floor for the safety against horizontal forces in all directions.
In regard to installation of a brace, if connecting portions at both ends of the brace loosen, the brace fails to function as a brace, and in the case where the brace is used in a wall that withstands a large horizontal load, design and construction of the connecting portions are complicated. Therefore, in order to ensure that the construction is performed properly, a method in which, instead of the brace or in combination with the brace, a bearing wall panel is nailed to the frameworks for reinforcement has been employed.
In a building, a wall that has the capability to resist a horizontal load “i.e. a lateral force” such as that from an earthquake or wind is referred to as a bearing wall, and a wall that is not structurally fixed is referred to as a non-bearing wall.
Moreover, in a wooden building, a wall that resembles the bearing wall but that is imperfectly fixed and has a low resistance “e.g., a partition wall or the like” is referred to as a semi-bearing wall.
Since connecting portions of a wooden building are easily rotated, it is not possible for the building to resist the horizontal load such as that from an earthquake or wind only with pillars and beams. For this reason, it is required to provide a predetermined amount of bearing walls on each floor. A building with many bearing walls has excellent resistance to earthquakes and wind. Furthermore, the earthquake resistance can be enhanced when various members of the building are properly bound together with metal fittings.
The bearing wall can be produced by attaching a brace to a framework with metal fittings or securing a bearing wall panel composed of a board such as structural plywood to a framework with predetermined nails. On the other hand, a wall in which only a moisture-permeable waterproof sheet or a siding is attached to a framework is not a bearing wall.
An example of a numerical value representing the performance of a bearing wall is a wall strength factor. A wall strength factor of 1.0 times indicates the ability to resist a horizontal load “i.e. a lateral force” of 1.96 KN per meter of wall length. The higher this value, the higher the performance and the larger the horizontal load the bearing wall can withstand. With respect to the timber framework construction method, Article 46 of the Order for Enforcement of the Building Standard Law and the Notification No. 1100 of the Ministry of Construction prescribe that the wall strength factor for several specifications of bearing walls be fall within a range of 0.1 to 5.0.
In regard to the earthquake resistance of a house, a seismic force acts on the center of gravity of the house, and the house deforms in a horizontal direction and also rotates on the center of rigidity. Therefore, if the center of gravity and the center of rigidity are too far from each other, excessive deformation occurs in part of the house, resulting in damage to structural members. As a result, load-bearing capacity of the house decreases, and the load of the seismic force is concentrated on the other portions, which may lead to collapse of the house in the worst case. Therefore, it is preferable that the center of gravity and the center of rigidity of the house coincide with each other.
Here, the center of gravity is the center of a planar shape of a building and is the center of the weight of the building. The center of rigidity is the center of forces that counteract a horizontal force and is the center of rigidities of bearing walls. The center of rigidity can be determined from horizontal rigidities of earthquake-resistant elements such as bearing walls and their coordinates. Furthermore, a discrepancy between the center of gravity and the center of rigidity of a building is defined by an eccentric distance and an eccentricity. The eccentricity that can be calculated from the eccentric distance is the ratio of the distance between the center of gravity and the center of rigidity to torsional resistance.
The center of gravity on each floor of the building can be calculated from an axial force due to sustained loading that occurs in principal members in terms of structural resistance, such as pillars that support vertical loads, and coordinates X, Y of those members. However, in the case of the timber framework construction method, it is supposed that the centroid of a plane coincides with the center of gravity assuming that the dead load and the live load on each floor are uniformly distributed in a plane and there is no imbalance. The center of rigidity can be calculated from horizontal rigidities of the earthquake-resistant elements such as bearing walls in each direction of calculation and their coordinates. Here, the horizontal rigidity can be calculated from the actual wall length and the wall strength factor, and the eccentricity can be calculated from the above-described center of gravity and center of rigidity.
Even when sufficient bearing walls are secured, there is a risk that the building may be deformed or twisted when an earthquake occurs, leading to collapse of the building, unless the bearing walls are arranged in a well-balanced manner without being concentrated on one side of the building. Generally, a building having many bearing walls in the vicinity of the periphery thereof is resistant to torsion. On the other hand, a so-called U-shaped arrangement in which, for example, the north side is fully constituted by bearing walls and the south side is fully constituted by openings is susceptible to torsion and can easily lead to the collapse when an earthquake occurs.
An example of a value representing the imbalance of bearing walls is the eccentricity. The larger the value of eccentricity, the larger the imbalance of bearing walls it represents. In the Notification No. 1352 of the Ministry of Construction in 2000, it is prescribed that the eccentricity of a wooden building specified by Article 46, Section 4 of the Order for Enforcement of the Building Standard Law should be 0.3 or less, and generally, it is said that a house whose eccentricity is 0.15 or less is particularly preferable.
As described above, in order to build an earthquake-proof building, it is necessary to provide a bearing wall. Conventionally, in the case of building a house using the timber framework construction method, a plate-like body referred to as a bearing wall panel has been used instead of a brace or in combination with a brace to form a bearing wall that counteracts a force acting in the horizontal direction such as that from an earthquake, wind pressure, or the like.